What is the minipill?

The progestin-only birth control pill, commonly called the minipill, is an oral contraceptive. Each pill contains a small amount of progestin, the synthetic form of the hormone progesterone. The pills come in packs of 28, and you take one pill every day so that you're getting a steady dose of the hormone.

Some women find progestin-only pills trickier to use than combination birth control pills (the Pill) because the minipill must be taken at the same time each day. (The combination pill, which contains both estrogen and progestin, is more forgiving.) The minipill is a good alternative for women who want to use oral contraceptives but who are breastfeeding or can't take estrogen for some other reason.

How does the minipill prevent pregnancy?

Progestin-only pills suppress ovulation in about half of the menstrual cycles of the women who take them. But they prevent conception primarily by causing changes in the cervical mucus. The mucus thickens, making it difficult for sperm to get through the cervix. So even if you do ovulate, sperm won't be able to make their way to your fallopian tubes to fertilize the egg. What's more, progestin may prevent pregnancy by thinning the lining of your uterus, making it less likely that an egg would implant there even if one did get fertilized.

The minipill contains less progestin than the combination pill, and the hormone's effect on your cervical mucus lasts only about 24 hours. That's why you have to be so careful about taking the minipill at the same time every day.

A newer progestin-only pill that prevents ovulation in up to 99 percent of cycles and requires a less stringent schedule is available in Europe and some other parts of the world, but it won't be in the United States anytime soon: The manufacturer hasn't yet applied for FDA approval.

Can I take the minipill if I'm breastfeeding?

Yes. Unlike the combination pill, progestin-only pills are considered compatible with breastfeeding by a host of organizations, including the Centers for Disease Control, the World Health Organization, the American Academy of Pediatrics, the American College of Obstetricians and Gynecologists, and Planned Parenthood Federation of America.

Only small amounts of progestin pass into your breast milk, and research to date shows no adverse effects from the minipill on a baby's weight gain, health, or development. Most studies show no change in the amount of breast milk produced by moms on progestin-only contraceptives, and some studies even show a small increase in milk supply.

How effective is the minipill?

If you're exclusively nursing your baby (meaning that your baby is taking in no formula or solid food), still haven't had a period, and are taking the minipill as directed, it's nearly 100 percent effective during the first six months after giving birth.

For women who aren't breastfeeding, the minipill is 91 to 99percent effective when taken consistently and correctly, which means that between one and nine women out of 100 who take it properly will get pregnant in the first year. That makes it less reliable than combination birth control pills, which are estimated to be about 99.7 to 99.9 percent effective with perfect use.

If you don't take the minipill exactly as prescribed – for example, you miss a pill or even fail to take them at the same time each day – the risk that you'll get pregnant is much higher, especially if you're not breastfeeding. And the more mistakes you make, the higher the risk. So while the minipill is a reliable method of birth control for women who never miss any pills and follow the instructions to the letter, it's not the best method for everyone.

What if I decide I want to get pregnant again?

All you have to do to reverse the effects of the minipill is to stop using it. You don't have to finish your pill pack unless you want to. Consider yourself fertile 24 hours after you take your last pill.

Some practitioners recommend using a barrier method of contraception, such as condoms or a diaphragm, and waiting until you have a couple of normal periods before trying to conceive because this can help you establish a more accurate due date. Others will give you the go-ahead to start trying right away if you want to. If you do get pregnant before your periods become regular again, don't worry — you can have an early sonogram to date your pregnancy.

(Note: Taking folic acid or a prenatal vitamin at least a month before you start trying to conceive will help prevent certain birth defects. That means beginning while you're still taking the minipill.)

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